As reviewed, e.g., by J. F. Hinsley, Non-destructive Testing, Macdonald & Evans, 1959, the field of nondestructive testing and evaluation includes a variety of methods as based on physical effects such as, e.g., radiological, acoustic, and magnetic interactions with test objects. More specifically, and as particularly germane to the invention, acoustic or ultrasonic methods involve the monitoring of an elastic wave as influenced by flaws or inhomogeneities in a test object; see, e.g., B. Banks, Ultrasonic Flaw Detection in Metals, Prentice-Hall, 1962.
For testing purposes, an elastic wave may be generated by means of an electro-acoustic transducer based on electromagnetic, electrostatic, magnetostrictive, or piezo-electric effects; also, waves have been generated optically as described, e.g., by
C. A. Calder et al., "Noncontact Material Testing Using Laser Energy Deposition and Interferometry", Materials Evaluation, Vol. 38 (1980), No. 1, pp. 86-91 (where, also, monitoring by interferometry is described) and by
C. P. Burger et al., "Laser Excitation Through Fiber Optics in NDE", Journal of Nondestructive Evaluation, Vol. 7 (1987), pp. 57-64, the latter disclosing laser energy as transmitted to a surface of interest via a flexible optical-fiber element.
Monitoring by interferometry involves the use of two coherent beams of light: an object beam which passes through--or is reflected by--an object under observation, and a reference beam which is unaffected by the object. Superposition of the two beams results in interference and, in a wide-aperture (full-field) system, the resulting intensity distribution yields an interferometric fringe pattern representing a contour map of constant optical path or optical phase difference.
While, typically in a laboratory setting, interferometric monitoring can be carried out with line-of-sight radiation, industrial settings may require testing in confined spaces and at hidden surfaces. Accordingly, it is a purpose of the invention to provide for a test device and method for detecting and characterizing flaws, such device and method being particularly suited with respect to ease of access to test objects in commercial practice.